The present invention relates to differential pressure indicators which may for example be used to give an indication of the state of a filter element in a filter by indicating the value of the pressure differential across it. An object of the invention is to provide an indicator which at all times give a definite and unambiguous indication in which the need to bring a shaft or like element out through a wall of the indicator through some kind of seal with its attendant friction, is avoided.
According to the present invention there is provided a differential pressure indicator comprising a piston slidable in a cylindrical sleeve the opposite ends of which are in communication with fluid spaces between which a differential pressure may exist, resilient biasing means urging the piston towards the end of the sleeve associated with the higher pressure, a resiliently yieldable stop means for yieldably impeding movement of the piston in the direction against the resilient bias, an indicator member slidable on a surface exteriorly the cylindrical sleeve and having a magnetic interconnection with the piston, a casing having a transparent portion and an opaque portion which conceals at least part of the indicator member in at least part of its range of movement, the arrangement being such that in use, with increasing differential pressure across the differential pressure indicator, the yieldable stop means holds the piston in a first position for as long as the differential pressure does not exceed a predetermined value but when the differential pressure exceeds this value the differential pressure across the piston will move the latter to a second position by causing the stop means to yield, the indicator member following the movement of the piston and co-operating with the casing so as to give a distinctive different visual indication in each of the positions of the piston.
Where required, there may be a plurality of spaced yieldable stops for yieldably impeding movement of the piston in the direction against the resilient bias. Thus, in a particular advantageous application of the invention, the differential pressure indicator may be used to indicate the degree of contamination of a filter element in a filter for filtering fluid, the filter including a bypass valve arranged to open and bypass the filter element when the differential pressure across the filter element reaches a predetermined value. The differential pressure indicator would then have two spaced yieldable stops thus defining three possible positions for the piston and thus for the indicator member. The first position would then indicate that the pressure differential across the filter element is normal and that the degree of contamination of the filter element is sufficiently small to ensure reliable operation. When, however, the degree of contamination increases to an extent such that the filter element, although still operating satisfactorily, needs to be replaced shortly, the pressure differential across the piston will be sufficient to move the piston pass the first stop, thereby moving the indicator member to a position indicating that the filter element should be changed at some convenient time in the near future. If, however, this is neglected, the contamination will gradually increase until the filter element is effectively blocked and the bypass valve accordingly opens. This high value of differential pressure is sufficient to move the piston past the second stop to carry the indicator member to a position indicating that the filter element must be changed immediately to avoid damage to the system. The pressure differential indicator may include means for operating an alarm signal under these conditions.
The transparent portion of the casing may be formed as a lens arranged to ensure that the same portion of the indicator member is seen over a wide range of viewing angles. The differential pressure indicator may be self-resetting, in which case the biasing means will return the piston to its original position whenever the differential pressure drops to a sufficiently low value. If desired, however, the indicator can be constructed so that the piston cannot return to a previous position without the operation of a resetting member which in the case of a filter may be accessible only when the filter element has been removed.